19.2 Oncogenes - Detailed Study Notes

Definition of Oncogenes: Specific genes that possess the capability to initiate cancer. They typically arise when normal cellular genes, known as proto-oncogenes, become overactive or dysregulated.
Discovery via Tumor Viruses: Oncogenes were first identified through the study of oncogenic (tumor) viruses. These viruses carry specifically designated genes, called $v-onc$ genes, which drive the uncontrolled proliferation of infected cells.
Universal Principle of Activation: While originally found in viruses, a virus is not required to cause cancer. Cellular proto-oncogenes can be activated through various non-viral mechanisms, including: - Mutation: Changes in the DNA sequence. - Gene Amplification: An increase in the number of copies of a gene. - Chromosomal Translocation: The rearrangement of parts between non-homologous chromosomes.

Mechanisms of Transformation: Some retroviruses have the ability to convert normal fibroblasts (cells found in connective tissue) into transformed, tumor-like cells.
Rous Sarcoma Virus (RSV): A classic example of a transforming retrovirus discovered in chickens.
Comparison: RSV vs. ALV (Avian Leukosis Virus): - Similarities: Both RSV and ALV are capable of infecting chicken embryo fibroblasts and replicating normally. - Key Difference: Only RSV induces cell transformation. This is because RSV carries an additional oncogene known as $v-src$ , whereas ALV lacks an oncogene ( $ALV \rightarrow \text{infection without transformation}$ ).
Conclusion from Retroviral Studies: Research into these viruses revealed that cancer can stem from the overactive forms of normal cellular genes that have been essentially \"captured\" and mutated by viruses.

Retroviral Genome Components: Standard retroviruses like ALV consist of genes required for replication but not transformation: - gag: Encodes structural proteins of the virus particles and the viral protease. - pol: Encodes reverse transcriptase and integrase. - env: Encodes envelope glycoproteins.
The src Oncogene: RSV contains these three genes plus the $src$ gene ( $v-src$ ), which encodes the Src tyrosine kinase. - Origin: RSV acquired a mutated version of the $src$ gene from chickens (a cellular proto-oncogene) and now carries it as an oncogenic version that is constitutively active.

Proto-oncogene Definition: Normal cellular genes that regulate cell growth, survival, or signaling. They are the ancestral versions of oncogenes.
Pathways to Oncogenesis: Proto-oncogenes become oncogenic when they are mutated, amplified, or misregulated so they no longer respond to normal cellular controls.
Case Study: The Raf Oncogene: - Raf Proto-oncogene Protein Structure: Consists of an amino-terminal ( $N$ -terminal) regulatory domain and a carboxy-terminal ( $C$ -terminal) protein kinase domain. - Viral Raf Oncogene ( $v-raf$ ): In this version, the regulatory domain is deleted and replaced by partially deleted viral Gag sequences ( $Δ \text{Gag}$ ). - Result: The loss of the regulatory domain causes the Raf kinase domain to become constitutively active (always turned on), which drives cell transformation.

Non-Viral Origins: Most human oncogenes arise through somatic mutations rather than viral infection.
Gene Transfer Experiments (Cooper & Weinberg): These experiments provided evidence for human oncogenes by extracting DNA from a human bladder carcinoma and introducing it into recipient mouse fibroblasts. - Observation: Some mouse cells underwent transformation after the integration and expression of the human tumor oncogene.
The Ras Gene Family: One of the most frequently mutated gene families in human tumors. - Point Mutation Example (Bladder Carcinoma): A single nucleotide base change converts the codon $GGC$ (encoding Glycine or $\text{Gly}$ ) to $GTC$ (encoding Valine or $\text{Val}$ ) at codon $12$ . - Effect: This creates the constitutively active hRas oncogene, which drives uncontrolled cell proliferation.

Altered Expression via Translocation: Translocations can move a proto-oncogene under the control of an inappropriate, highly active promoter or enhancer, leading to high levels of normal protein. - Burkitt’s Lymphoma: The $c-myc$ (cellular myelocytomatosis) proto-oncogene is translocated from chromosome 8 to the immunoglobulin heavy-chain (IgH) locus on chromosome 14 ( $t(8;14)$ ). This leads to constitutive high expression of $c-myc$ , a transcription factor promoting proliferation through pathways like PI3K/Akt and Cyclins D and E.
Fusion Proteins via Translocation: Translocations can fuse two genes to create a chimeric protein. - Chronic Myeloid Leukemia (CML): The $abl$ proto-oncogene on chromosome 9 translocates to the $bcr$ locus on chromosome 22, forming the Philadelphia chromosome. The resulting Bcr/Abl fusion protein has constitutive tyrosine kinase activity.
Gene Amplification: This mechanism increases the expression of otherwise normal proto-oncogenes. - erbB-2 (HER2): A receptor tyrosine kinase (RTK) often amplified in breast cancers. Overexpression drives excessive growth signaling through Ras, PI3K, and STAT.

Mechanisms of Hyperactivation: Oncogene proteins mimic or hyperactivate growth-control pathways. They can function as: - Growth Factors: e.g., EGF. - Growth Factor Receptors: e.g., ErbB, HER2. - Intracellular Signaling Proteins: e.g., Ras, Raf, MEK. - Transcription Factors: e.g., Fos, Jun, Myc.
The ERK Signaling Pathway Axis: A central oncogenic pathway consisting of $Ras \rightarrow Raf \rightarrow MEK \rightarrow ERK$ . - Function: Activated ERK enters the nucleus and induces the transcription of genes such as $fos$ that promote proliferation.
Mechanism of Tel/PDGFR Oncogene: In this oncogene, the extracellular domain of the Platelet-Derived Growth Factor Receptor (PDGFR) is replaced by the amino-terminal sequences of the Tel transcription factor. - Activation: The Tel domain includes a dimerization domain that causes the receptor to self-dimerize and activate without its ligand (ligand-independent activation).

AP-1 Transcription Factor: Formed by the dimerization of Fos and Jun proteins. It is a key transcriptional effector that is often induced downstream of the Ras-ERK pathway. - Role: AP-1 activates the transcription of cyclin D1 and other growth-inducible genes.
Oncogenes and Cell Survival (Apoptosis Blocking): Blocking apoptosis is a critical mechanism of oncogenic action. - The PI 3-kinase/Akt Pathway: Activated by growth factors and receptors. - Akt Function: Akt acts as a survival effector by inhibiting proapoptotic proteins like Bad and FOXO. - Result: Inhibition of Bad/FOXO leads to the activation of Bcl-2 family antiapoptotic proteins, which prevents the release of Cytochrome-c and inhibits cell death.